Semiconductor manufacturers are continually working to reduce time-to-market in an effort to improve serviceability while decreasing operating costs. This is particularly true for fabrication facilities that produce high volumes (on the order of 1,000 wafer starts per day) of complex products. As such, efforts have been made to develop and implement systems and methods, such as lean manufacturing and continuous flow manufacturing, to improve manufacturing process performance.
Range management is a production control methodology that employs lean manufacturing principles. In range management systems, wafer processing is partitioned into a series of flows, where the products in each flow follow similar routes through the fabrication facility. These flows are divided into ranges, and each range may include one or more operations. An operation is where the units (e.g., wafers) of a production lot are actually worked on, and may include, for example: lithography, metrology, overlay, metrology CD, etc. Normally, a particular lot spends twenty-four hours (e.g., a range day) in a range, and then is moved to the next range. Of the twenty-four hours, the lot may, for example, undergo six hours of processing time in the operations of the range, sixteen hours of non-processing time waiting in a queue, and two hours of transit time being moved between operations and/or other ranges. Thus, it is possible for numerous lots to be processed through a single range a twenty-four hour period.
Typically, new production lots are started every day, and a particular lot may undergo processing in seventy or more ranges before it is complete. Also, different lots may result in different end products, thereby requiring different operations during their manufacture. As such, there may be thousands of active lots in the fabrication facility at any one time, with those lots being dispersed amongst the numerous operations of the fabrication facility. Moreover, a given operation is not necessarily unique to a particular range. The same operation may be included in different ranges in the same flow and/or different ranges of different flows. As a result, it is common for plural lots to simultaneously require processing in the same operation, thereby necessitating that some lots wait in a queue while another lot is processed in the operation.
Range management systems are used to manage the flow of the production lots through the fabrication facility. Range management systems focus on delivering work in progress (WIP) (e.g., lots or units in a lot) on a known route at a known speed to meet committed customer deliveries on schedule. This is accomplished, at least in part, by creating daily production targets for each range and then controlling the fabrication facility by managing the WIP in the individual ranges.
Disruptions in product flow (e.g., a broken machine or tool), however, can cause elevated WIP levels in ranges. Conventional range management systems constrict product flow into ranges with elevated WIP in order to keep product cycle times predictable. Flow is constricted by “stopping” a range that is directly upstream of a range with elevated WIP. The effect of “stopping” a range is that none of the WIP in the stopped range will undergo any processing during the range day. While range stops are necessary for flow control for keeping product cycle times predictable, factory resources are not fully utilized when WIP is stopped.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.